1. The problem statement, all variables and given/known data To measure the activity of a rock thought to be radioactive, a physicist puts the rock beside a detector and counts 225 particles in 10 minutes. To check for background, she removes the rock and then records 90 particles in 6 minutes. She converts both these answers into rates, in particles per hour, and takes their difference to give the activity of the rock alone. What is her final answer, in particles per hour, and what is its uncertainty? Does she have significant evidence that the rock is radioactive? 3. The attempt at a solution With stone she measures 1350 particles and hour and without the stone 900, so the rock alone should be 400 particles an hour. How do you calculate the uncertainty in this case? It seems to me calculating the standard deviation wouldn't work, because they are two entirely different measurements (with rock and without rock).